306 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 



in a given substance an allotropic change. Thus, when 

 a piece of iron is placed in nitric acid in contact with 

 platinum, the iron becomes less electro-positive, or, what 

 is the same thing, more electro-negative, than it was be- 

 fore, and the acid can no longer oxidize it. The contact 

 of the very same substance, platinum, determines an an- 

 alogous change in chlorine giving it at once the capac- 

 ity of uniting with hydrogen. The porous condition of 

 spongy platinum is not essential to the result, for clean 

 platinum foil exhibits the same phenomenon. 



In the case of iron, the action of a high temperature 

 or the contact of platinum throws the metal from the 

 active to the passive state ; in the case of chlorine the 

 same causes apparently produce the opposite result, 

 throwing the gas from the passive to the active state. 

 But the difference is rather in appearance than in real- 

 ity. In both cases it amounts to the same thing, and is 

 an exaltation of the electro-negative qualities of either 

 substance respectively. 



The same causes, therefore, which produce allotropic 

 changes in other bodies produce analogous changes in 

 chlorine. 



Now, among the physical facts connected with the 

 theory of types and substitutions, two are prominent : 

 1st. The union of chlorine with hydrogen, giving rise to 

 the removal of that hydrogen as hydrochloric acid. 2d. 

 The subsequent function discharged by the chlorine, 

 which has entered as an integrant portion of the mole- 

 cules, and occupies the place of the hydrogen removed. 

 This function is in many instances that of the hydrogen 

 itself, and it is this fact which is the remarkable point in 

 the phenomena of substitution that an intensely elec- 

 tro-negative body can act the part of a positive body. 

 It is this fact which is leading chemists to the conclusion 

 that the properties of compound bodies arise as much 



